1. Field of the Invention
The present invention relates to an image forming apparatus for forming an optical image, based on electrical image information. More particularly, the invention concerns an image forming apparatus for forming a desired image at high light utilization efficiency by using a coded phase image as an input image and giving a phase shift to a Fourier light image of the coded phase image.
2. Related Background Art
A conventionally known method for microscopic observation of an object having differences of phase is the so-called Zernike phase contrast method for emphasizing the small differences of phase in microscopy and proposals to extend this method to the field of image display and the like are made in References 1 to 3 below by Jesper Gluckstad.
Reference 1; "Generalized Phase Contrast Imaging," Patent Application WO 96/34307 (Oct. 31, 1996)
Reference 2; "Adaptive array illumination and structured light generated by spatial zero-order modulation in a Kerr medium," Optics Communications 120 (1995) pp. 194-203
Reference 3; "Phase contrast image synthesis," Optics Communications 130 (1996) pp. 225-230
For example, as disclosed in Reference 3, the input image is coded into a phase-modulated light image by a phase modulation type spatial light modulator and the phase-modulated light image is displayed thereon and then is guided through a 4f lens configuration to form an output image on an output plane. At this time, a phase contrast filter is placed on the Fourier plane of the 4 f lens configuration to shift the phase of the zero-order light component on the Fourier plane, thereby converting the input image to the output image (as will be referred to as a target image) of desired intensity.
Use of this phase shift image display (formation) method makes it theoretically possible to form the output image of desired intensity at the light utilization efficiency of 100%. Therefore, much higher light utilization efficiency is achieved, when compared with the light utilization efficiency of about 20% in the case of a normal liquid-crystal television using a polarizing plate as a display panel screen.
An example of the phase modulation type spatial light modulator known is a Parallel Aligned Liquid-crystal Spatial Light Modulator (PALSLM) as described in Reference 4; Tsutomu Hara, "Spatial light modulator and optical analog arithmetics," OplusE, Special Issue: Optoelectronic image processing technology, March 1995, No. 184, pp. 101-108, published by Shin Gijyutsu Communications Kabushiki Kaisha. This Reference 4 reports an example to modify the PALSLM, originally, of an optical address type to that of an electrical address type by combination with a display screen of CRT. With this modified phase modulation type spatial light modulator, a desired input image is displayed on the CRT and the input image is guided to the PALSLM, thereby enabling application of computer system.
Further, Reference 5; Hiroshi Imai et al., "Circular polarization phase modulation characteristics of TN liquid-crystal panel," Optics Vol. 21, No. 8 (August 1992) pp. 42-46, presents a proposal to utilize a cheap liquid-crystal television of high resolution as a phase modulation type spatial light modulator. This proposal also enables introduction of computer system, because a desired input image can be displayed on the liquid-crystal television.
The above-stated proposals in References 1 to 3 can achieve desired results, if various characteristics of optical system are ideal. In practice, however, there exists noise, for example, resulting from nonuniformity of laser light, MTF of optical system, nonuniformity of spatial light modulator, degradation of accuracy of phase contrast filter, temperature characteristics of each optical element, and so on. This noise sometimes made nonuniform the contrast of the output image obtained by the real optical system, different from the ideal state.
In applications of the spatial light modulator of combination of CRT with PALSLM in aforementioned Reference 4, image distortion of CRT is normally about 3%, which would be a serious problem in uses not permitting this distortion, for example, in computer graphic holography (CGH) technology, high-precision laser processing technology, optical filtering technology, optical modulation technology for separately controlling the phase and the amplitude of light, and use of stack of plural spatial light modulators.
In applications of the liquid-crystal television in aforementioned Reference 5, since the liquid-crystal television has the pixel structure, the maximum aperture ratio thereof is 50% or so and optical diffraction takes place at edges of pixel driving electrode thereof, posing a problem of large optical loss. For example, as shown in FIG. 24A, laser light emitted from laser light source 2 is expanded into parallel light by collimator lens 4 to irradiate liquid-crystal television 6, and image light displayed on the liquid-crystal television 6 is focused by Fourier lens 8 to form an image on screen 10 placed at the position of the focal point of the Fourier lens 8. Then, convolution noise due to diffraction at each pixel of liquid-crystal television appears on the screen 10, as shown in FIG. 24B. The intensity of the zero-order light component (the Fourier image appearing at the optic center) is reduced to a fraction, because of appearance of diffraction images around it. Therefore, there arises a problem that an image obtained by inverse Fourier transform of the zero-order light component is of low luminance and becomes unsharp.
The present invention has been accomplished in view of the problems in the conventional technology as described and an object of the present invention is to provide an image forming apparatus for forming a sharp image at high light utilization efficiency.